Summary

Amenorrhoea is the transient or permanent absence of menstrual flow and may be subdivided into primary and secondary presentations, relative to menarche.
[1]Herman-Giddens ME, Slora EJ, Wasserman RC, et al. Secondary sexual characteristics and menses in young girls seen in office practice: a study from the Pediatric Research in Office Settings network. Pediatrics. 1997;99:505-512.
http://www.ncbi.nlm.nih.gov/pubmed/9093289?tool=bestpractice.com

Primary amenorrhoea: lack of menses by age 15 years in a patient with appropriate development of secondary sexual characteristics, or absent menses by age 13 years and no other pubertal maturation.

Secondary amenorrhoea: lack of menses in a non-pregnant female for at least 3 cycles of her previous interval, or lack of menses for 6 months in a patient who was previously menstruating.

Although overlapping attributes exist between the two groups, the diagnostic approaches vary significantly. Primary amenorrhoea is rare: about 0.3% of women fail to establish menstruation.
[2]Kiningham RB, Apgar BS, Schwenk TL. Evaluation of amenorrhea. Am Fam Physician. 1996;53:1185-1194.
http://www.ncbi.nlm.nih.gov/pubmed/8629565?tool=bestpractice.com
The prevalence of primary amenorrhoea in the US is <0.1%, compared with 4% for secondary amenorrhoea.
[3]Timmreck LS, Reindollar RH. Contemporary issues in primary amenorrhea. Obstet Gynecol Clin North Am. 2003;30:287-302.
http://www.ncbi.nlm.nih.gov/pubmed/12836721?tool=bestpractice.com
[4]Pettersson F, Fries H, Nillius SJ. Epidemiology of secondary amenorrhea: incidence and prevalence rates. Am J Obstet Gynecol. 1973;117:80-86.
http://www.ncbi.nlm.nih.gov/pubmed/4722382?tool=bestpractice.com
Even when causes of primary and secondary amenorrhoea overlap, the relative likelihoods of these aetiologies may differ. For example, polycystic ovary syndrome may cause either primary or secondary amenorrhoea, but much more commonly causes secondary amenorrhoea. Many causes of primary amenorrhoea are rare in the general population (e.g., Kallman syndrome). Conditions that may seem to be rare events generally may appear more commonly in this subgroup of adolescent girls presenting with primary amenorrhoea.